Polarization rotation technique for use with two dimensional TEM mode lenses

ABSTRACT

A non-dispersive, parallel-plate high-frequency lens-antenna has dipole elements which allow it to radiate or receive TEM mode electromagnetic energy polarized with the direction of polarization parallel to the plane of the lens.

United States Patent Lewis 1 Nov. 25, 1975 POLARIZATION ROTATIONTECHNIQUE FOR USE WITH TWO DIMENSIONAL TEM MODE LENSES [75] Inventor:Bernard L. Lewis, Oxon Hill, Md.

[73] Assignee: The United States of America as represented by theSecretary of the Navy, Washington, DC.

22 Filed: 0a. 18, 1974 211 Appl.No.:5i6,244

[52] US. Cl. 343/754; 343/756; 343/909 [51] Int. Cl. .,H01Q 15/04; H01015/08; H010 i9/08 [58] Field of Search 343/727, 753, 754, 756, 343/799,800, 909, 911 R [56] References Cited UNITED STATES PATENTS 2508.0845/[950 Alford 343/727 Carson 4. 343/754 Brickey 343/756 PrimaryE.mminerPaul L. Gensler Attorney, Agent, or FirmR. S4 Sciascia; ArthurL, Branning; Norman V, Brown [57! ABSTRACT A non-dispersive,parallel-plate highirequency lensantenna has dipole elements which allowit to radiate or receive TEM mode electromagnetic energy polarized withthe direction of polarization parallel to the plane of the lens.

4 Claims, 3 Drawing Figures U.S. Patent Nov. 25, 1975 POLARIZATIONROTATION TECHNIQUE FOR USE WITH TWO DIMENSIONAL TEM MODE LENSESBACKGROUND OF THE INVENTION The present invention relates tohigh-frequency an tennas. and more particularly to parallel-plate lensantennas. Parallelplate lens-antennas are generally utilized as twodimensional electromagnetic lenses to focus high-frequencyelectromagnetic energy into an antenna beam. Lenses of this type formthe equivalent of a cylindrical optical lens and are especially usefulin producing a radar beam having a beamwidth narrow in azimuth but largein elevation.

Prior non-dispersive parallel-plate TEM lens-antennas, although capableof radiation energy polarized with the direction of polarization normalto the plane of the lens, have heretofore been incapable of radiatingenergy polarized with the direction of polarization parallel to theplane of the lensa highly desirable polarization for many applications.

In order to transmit energy polarized parallel to the plane of the lens,prior parallel-plate antennas have had to be operated in the dispersive,non-TEM mode. Operation in any dispersive mode causes propagation inwhich the velocity of propagation through the lens is a function offrequency, termed dispersive propagation. Dispersive propagation isundesirable because it causes different frequency components of thesignal to focus at different points, resulting in an objectionableelectromagnetically fuzzy" focusing known as chromatic aberration.

The present invention utilizes a non-despersive parallelplatelens-antenna to transmit and receive TEM mode electromagnetic energypolarized in the same direction as the plane of the lens.

SUMMARY OF THE INVENTION DESCRIPTION OF THE DRAWINGS FIG. I shows in topview an embodiment of the present invention.

FIG. 2 shows in side view the embodiment of the present inventiondepicted in FIG. 1.

FIG. 3 depicts addition of quarter-wavelength chokes to a parallel-platelens-antenna (shown incomplete, with only selected elements presented inthe interest of clarity).

DETAILED DESCRIPTION OF THE INVENTION A parallel-plate lens-antenna 10,designed to radiate electromagnetic energy of wavelength A, is indicatedgenerally in FIGS. 1 and 2. The lens-antenna has circular shaped upperand lower plates I2 and 14 arranged in a parallel stacked fashion. Eachplate 12, I4 has a diameter, D, of one wavelength A or greater. Theplates I2, I4 are constructed in conventional manner from any goodconductor, such as aluminum, and are sufficiently thick and rigid so asto provide structural support for the lens assembly. The spacingdistance, t, between the parallel plates 12, I4 is chosen to be lessthan Between and adjacent to the parallel-plates 12, I4 is a taperedlow-loss dielectric material 16 having a dielectric constant, commonlytermed k, of approximately 3.5. The dielectric material 16 is in theform of a circular disc with sloping sides in which the upper radius issmaller than the lower radius, thus forming a tapered region 18 at thedisc periphery. The tapered region l8 typically has a linear taper. withthe difference between upper and lower radii of approximately Aplurality of conventional feed elements 26 adapted to radiate or receivehigh-frequency electromagnetic energy are attached to the upper plate12. Each feed element 26 has a coaxial connector section 28 fastened tothe upper plate 12, and an inner conductor element 30 which passesthrough an insulated aperture in the upper plate 12 and extends into theregion between the upper and lower plates I2, 14. The end of each innerconductor element 30 is located adjacent and above the center of taperregion 18.

The number of feed elements 26 utilized in the present invention isequal to where r is the radius from the center of the lens I0 to thefeed elements 26.

Conventional quarter-wavelength chokes 32 may optionally be utilized inthe embodiment of the present invention, as depicted in FIG. 3. Forclarity, FIG. 3 depicts only selected portions of elements shown in FIG.2, in addition to quarter-wavelength chokes 32. Each of the parallelplates 12, 14 has a choke 32 disposed on its outer surface near itsperiphery. Each choke 32 is formed by a first flat conducting section 34and a second conducting section 36. The first section 34 is shaped inthe form of a washer and is disposed parallel to the outermostquarter-wavelength of the corresponding upper and lower plates I2, 14.Each first section 34 is terminated and connected at its inner end tocorresponding second connecting section 36, shaped in the form of aring. Connecting sections 36 are disposed perpendicularly to therespective top and bottom plates I2, 14, to which they are respectivelyattached.

Construction of parallel plate lens-antennas in general is fullydiscussed in various texts such as Antenna Engineering Handbook", editedby H. Jasik, McGraw 3 Hill. l96l, (at page 2726) and in MicrowaveScanning Antennas, Volume I. beginning at p. 23], edited by R. C.Hansen, Academic Press, 1964.

Returning again to FIG. 1, a plurality of half-wave high-frequencydipoles 40 for radiating or receiving electromagnetic energy areattached to the periphery of upper and lower plates l2, 14. Thesedipoles 40 are at the heart of the present invention and are the meansby which rotation of the direction of polarization of theelectromagnetic energy is accomplished.

Each half-wave dipole 40 is formed from two tubular rods. An upper rod42 is attached to the upper plate 12 and a lower rod 44 is attached tothe lower plate 14. The upper rod 42 extends from the upper plate 12 ina downward and outward direction, and is bent at a point halfway betweenthe plates so as to be approximately one-quarter wavelength long andparallel to the tangent at the point of attachment to the upper plate12. The lower rod 44 is attached to a point on the lower plate 14located directly below the point of attachment of the upper rod 42.Lower rod 44 is bent in a shape similar to that of the upper rod 42, butin opposite directions, so as to result in an overall dipole structureonehalf wavelength long aligned parallel to the plane of theparallel-plates 12, 14.

In this embodiment of the present invention each dipole 40 is spacedaway (in the radial direction by onequarter wavelength, from theperipheral edges of the parallel-plates 12, 14, although in otherembodiments of the present invention this spacing may be other thanDipoles 40 described above are of the narrowband type. Alternatively,broadband type dipoles may be used. Typical of such broadband dipolesare the triangular shaped assemblies, commonly referred to as bowtie"dipoles. the dipoles 40 utilized in the present invention are ofconventional type, and their operation and construction is described indetail in many textbooks dealing with antenna theory, such as in thepreviously cited reference to Antenna Engineering Handbook" at page24.25 through 24.26.

The maximum number of dipoles 40 that may be utilized is determined bythat number of dipoles that can be fitted, without touching, around aperiphery approximately one-quarter wavelength larger than that of theplates 12, 14. A spacing between dipole centers of 0.7A has been foundto work well and is utilized in the embodiment of the present invention.

In operation, the circular design of the parallel-plate lens creates alocus of focal points along a focal circle near the periphery of thelens. It is about this focal circle that feed-elements 26 are spaced ina symmetrical fashion. high-frequency electromagnetic energy. typicallyl 3 Hz, propagates from the energy source (not shown) through variousswitching means and coaxial cable (not shown) to the connectors 28 ofselected monopole feed-elements 26. Each center conductor 30 acts as amonopole radiator which omnidirectionally radiates electromagneticenergy in the TEM mode.

It should be noted that although any feed-element 26 may be used totransmit or receive electromagnetic energy. this discussion will treatthe case of transmitting 4 only, since the lens-antenna system conceptsdiscussed herein are equally applicable to transmitting or receiving.Also, this discussion addresses operation of only one feed-element 26,although simultaneous operation of selected feed-elements 26 in eithertransmit or receive mode is possible.

Energy from feed-elements 26 propagates with the E field perpendicularto the plane of the lens-antenna (defined by the plane midway andparallel to the upper and lower plates l2, 14) as indicated by arrow 50.

Electromagnetic energy propagating from a feed-element 26 through thedielectric focusing section 16 (indicated by lines 52) is focused intoan antenna beam 53 (indicated by lines 54), and coupled to space throughthe dipoles 40. Antenna beam 53 has a beamwidth B arc sin For a typicalvalue of D 10A, the beamwidth is approximately 6. The manner in whichfocusing is accomplished by the dielectric section 16 in combinationwith the plates l2, 14 is wellknown, and is described, for example, inMicrowave Scanning Antennas edited by R. C. Hansen, Academic Press, 1964(at page 23] hereby incorporated by reference.

It should be noted that by choosing the spacing distance, t, between theparallel plates 12, 14 to be less than possible undesirable propagationof modes of higher order than the TEM (i.e. dispersive modes) isprecluded. Also, the choice of dielectric constant k 35 results in thefocal points (formed by the lens) being at the periphery of thedielectric section 16.

Energy propagating between the parallel plates l2, 14 causes the platesl2, 14 to have an associated charge distribution of opposite polarity(in any given period of time). [t is well-known that the polarization ofan electric field is in the direction from a positive chargedistribution to a negative charge distribution Since the plates l2, 14are oppositely charged, the energy between them has a direction ofpolarization normal to the plane of the lens 10 (as indicated by arrow50).

In the present invention, the charge distribution experienced by eachparallel plate l2, 14 causes the attached dipole section 42, 44 to alsoexperience the same charge distribution. Because of this, the sections42, 44 (lying in the plane of lens 10) of each dipole pair 40 havecharge distributions of opposite polarity (in any given period of time).Since all upper dipole sections 42 of each dipole 40 are disposed to thesame side (e.g. to the right or counterclockwise) of all lower dipolesection 44, the direction of polarization of the electric field whenradiated from the dipoles 40 is parallel to the plane of the lens 10 (asindicated by arrows 51). in this manner, the arrangement of dipoleelements 20 causes rotation of the direction of polarization from normalto the lens antenna 10 plane (while the energy is confined between theparallel plates 12, 14) to parallel to the lens plane upon radiation.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. [t is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of theUnited States is:

l. A lens-antenna for transmitting and receiving highfrequencyelectromagnetic energy polarized with the direction of polarizationparallel to the plane of the lens comprising:

an upper conducting plate;

a lower conducting plate substantially identical to said upper plate anddisposed parallel and below said upper plate, forming a focusing regionbetween said plates;

a dielectric section disposed between said upper and lower plates. saidsection having flat upper and lower surfaces in contiguous contact withsaid plates and filling a substantial portion of said focusing region;

means disposed within said focusing region for propagating and acceptinghigh-frequency electromagnetic energy from at least one selected pointwithin said focusing region; and

dipole means connected to said parallel plates for radiating andreceiving high-frequency electromagnetic energy, said dipole meansdisposed at the periphery of said parallel plates and arranged to havethe radiating sections of said dipole means lie parallel to the plane ofsaid parallel plate lens, one pole of said dipole means connected tosaid upper plate and the other pole of said dipole means connected tosaid lower plate;

whereby electromagnetic energy polarized with the direction ofpolarization normal to the plane of said lens is propagated by saidpropagating means, focused into a beam shape by said dielectric section,and rotated in polarization by and radiated by said dipole means,thereby radiating electromagnetic energy polarized with the direction ofpolarization parallel to the plane of said lens-antenna.

2. The lens-antenna of claim 1 wherein said plates are spaced apart byless than one wavelength divided by twice the square root of thedielectric constant of said dielectric section, whereby propagation ofonly TEM mode electromagnetic energy may be supported in the regionbetween said plates.

3. The lens antenna of claim 2 wherein the dielectric constant of saiddielectric section is 3.5.

4. The lens-antenna of claim 3 wherein said parallel plates have acircular form with a diameter greater than one wavelength, and whereinsaid dipole means are comprised of a plurality of dipole elements, eachsaid element having a length of one-half wavelength and wherein thecenters of said dipole elements are spaced apart by 0.7 wavelengths.

1' ll k l

1. A lens-antenna for transmitting and receiving high-frequencyelectromagnetic energy polarized with the direction of polarizationparallel to the plane of the lens comprising: an upper conducting plate;a lower conducting plate substantially identical to said upper plate anddisposed parallel and below said upper plate, forming a focusing regionbetween said plates; a dielectric section disposed between said upperand lower plates, said section having flat upper and lower surfaces incontiguous contact with said plates and filling a substantial portion ofsaid focusing region; means disposed within said focusing region forpropagating and accepting high-frequency electromagnetic energy from atleast one selected point within said focusing region; and dipole meansconnected to said parallel plates for radiating and receivinghigh-frequency electromagnetic energy, said dipole means disposed at theperiphery of said parallel plates and arranged to have the radiatingsections of said dipole means lie parallel to the plane of said parallelplate lens, one pole of said dipole means connected to said upper plateand the other pole of said dipole means connected to said lower plate;whereby electromagnetic energy polarized with the direction ofpolarization normal to the plane of said lens is propagated by saidpropagating means, focused into a beam shape by said dielectric section,and rotated in polarization by 90* and radiated by said dipole means,thereby radiating electromagnetic energy polarized with the direction ofpolarization parallel to the plane of said lens-antenna.
 2. Thelens-antenna of claim 1 wherein said plates are spaced apart by lessthan one wavelength divided by twice the square root of the dielectricconstant of said dielectric section, whereby propagation of only TEMmode electromagnetic energy may be supported in the region between saidplates.
 3. The lens antenna of claim 2 wherein the dielectric constantof said dielectric section is 3.5.
 4. The lens-antenna of claim 3wherein said parallel plates have a circular form with a diametergreater than one wavelength, and wherein said dipole means are comprisedof a plurality of dipole elements, each said element having a length ofone-half wavelength and wherein the centers of said dipole elements arespaced apart by 0.7 wavelengths.